The present invention relates to the reduction of sulphur content in FCC heavy gasoline.
There is increasing demand to reduce the sulphur content of gasoline in order to meet new requirements for low exhaust emissions. The largest contribution to sulphur in the gasoline pool comes from FCC gasoline. The sulphur content can be reduced by hydrotreating. However, hydrotreating results in saturation of olefin species in the FCC gasoline leading to unacceptable losses in Octane Number. Several processes have been proposed whereby the FCC gasoline is fractionated into a light (low boiling) fraction and a heavy (high boiling) fraction, and where only the heavy fraction is hydrotreated. The reason for doing this is linked to the distribution of sulphur and olefin species as a function of boiling point. As apparent from Table 1, most of the sulphur is found in the highest boiling approximately 30% of the FCC gasoline, whereas most of the olefins are found in the lightest approximately 70% of the FCC gasoline. By hydrotreating only the heavy fraction and blending the hydrotreated product with the untreated light fraction, the required degree of desulphurization can be obtained with moderate olefin reduction and moderate loss of Octane Number. However, the loss of Octane Number is usually unacceptably high.
TABLE 1 ______________________________________ Analysis of an FCC Gasoline Cumulative Boiling Liquid Liquid Olefins Range .degree. C. Vol. % vol. % S, wppm vol. % ______________________________________ IBP-50 2.1 21 3 48.6 50-75 18.2 39.2 178 59.7 75-100 10.6 49.8 219 46.2 100-125 11.4 61.2 565 34.8 125-150 13.2 74.4 633 22 150-175 8.3 82.7 576 12.6 175-200 9.3 92 580 9.4 200+ 8 100 3255 3.2 ______________________________________